Article pubs.acs.org/joc
Nickel Catalyzed Cross-Coupling of Aryl C−O Based Electrophiles with Aryl Neopentylglycolboronates Pawaret Leowanawat, Na Zhang, and Virgil Percec* Roy & Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States S Supporting Information *
ABSTRACT: The efficiency of mesylates, sulfamates, esters, carbonates, carbamates, and methyl ethers as C−O-based electrophiles attached to the 1- or 2-position of naphthalene and to activated and nonactivated phenyl substrates was compared for the first time in Ni-catalyzed cross-coupling with phenyl neopentylglycolboronates containing electron-rich and electrondeficient substituents in their para-position. These experiments were performed in the presence of four different Ni(II)- and Ni(0)-based catalysts. Ni(II)-based catalysts mediate the cross-coupling of most 2-naphthyl C−O electrophiles with both arylboronic acids and with neopentylglycolboronates when K3PO4 is used as base. The same catalysts are not efficient when CsF is used as base. However, Ni(0)-based catalysts exhibit selective efficiency, and when reactive, their efficiency is higher than that of Ni(II)-based catalysts in the presence of both K3PO4 and CsF. These results provide both reaction conditions for the crosscoupling, and for the elaboration of orthogonal cross-coupling methodologies of various C−O based electrophiles with aryl neopentylglycolboronates. With the exception of mesylates and sulfamates the efficiency of all other 2-naphthyl C−O electrophiles was lower in cross-coupling with aryl neopentylglycolboronates than with arylboronic acids
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INTRODUCTION Suzuki−Miyaura cross-coupling is one of the most important reactions used in the construction of C−C bonds. Pd is widely used as the catalyst in this reaction.1 Recently, Ni, which is less expensive and more reactive toward C−O-based electrophiles than Pd, has been employed as an alternative catalyst in Suzuki−Miyaura cross-coupling.2 Aryl sulfonates,3 ethers,4 esters,3i,5 carbonates,6 carbamates,6−8 sulfamates,6,7b,8 and phosphates9 have been elaborated as C−O-based electrophiles in cross-coupling with arylboronic acids, and for the case of aryl methyl ethers4 also with aryl neopentylglycolboronates, all under various Ni-catalyzed conditions. Since the first report on the Ni-catalyzed Suzuki−Miyaura cross-coupling of aryl sulfonates with arylboronic acids,3a interest in utilizing accessible and inexpensive C−O based electrophiles in cross-coupling continues to develop. Suzuki− Miyaura cross-coupling of aryl mesylates and tosylates with arylboronic acids using inexpensive Ni(II)-based catalysts is well established.3d However, in some situations, boronic esters are more favored than boronic acids in Suzuki−Miyaura crosscoupling reactions. For example, boronic esters can be used in stoichiometric amounts due to their monomeric species present under anhydrous conditions when boronic acids form anhydrides.10 At the same time, boronic esters are less sensitive © 2011 American Chemical Society
than boronic acids in some reactions such as protodeborylation.10 Boronic esters are prepared by the esterification of boronic acids11 and by transition-metal-catalyzed borylation of aryl halides.2,11 Transition-metal-catalyzed borylation provides an one-step synthesis of boronic esters.2 Our group is interested in the development of inexpensive Ni-catalyzed borylation reactions of aryl halides3g,12 and sulfonates13 and of their subsequent cross-coupling. Aryl neopentylglycolboronates are less expensive and more atom economic than the currently more commonly used pinacolboronates.12a The versatile Ni(II)-catalyzed borylation of aryl halides and sulfonates with neopentylglycolborane generated in situ from neopentylglycol and BH3·S(CH3)2 tolerates a variety of ortho-, meta- and paraelectrophilic functional groups and provides excellent yields.14,15 Some preliminary data suggested that Ni-catalysts can cross-couple aryl halides with aryl neopentylglycolboronates in moderate to good yields.12a Encouraged by these results, preliminary cross-coupling reactions between aryl neopentylglycolboronates and aryl sulfonates were also reported.3g A more comprehensive study on the cross-coupling Received: November 7, 2011 Published: December 28, 2011 1018
dx.doi.org/10.1021/jo2022982 | J. Org. Chem. 2012, 77, 1018−1025
The Journal of Organic Chemistry
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Table 1. Cross-Coupling of 2-Naphthyl-Containing C−O Electrophiles with p-Methoxyphenyl Neopentylglycolboronate Catalyzed by NiCl2(PCy3)2/ K3PO4 or CsF in Toluene at 110 °C
a
entry
OR
base
time (h)
yielda (%)
1 2 3 4 5 6 7 8 9 10 11 12
OMs OSO2NMe2 OPiv OBoc OCONEt2 OMe OMs OSO2NMe2 OPiv OBoc OCONEt2 OMe
K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 CsF CsF CsF CsF CsF CsF
12 12 12 24 24 24 12 12 12 24 24 24
25 54 6 0 34 13 7 1 0 0 0 0
Isolated yield.
dioxane at 130 °C3c and NiCl2(dppe) or NiCl2(dppp) both in toluene and in dioxane at temperatures between 80 and 100 °C.3d,k Ni(COD)2/PCy3/K3PO4/THF was shown to crosscouple aryl sulfonates with arylboronic acids at room temperature.3e Preliminary results on the cross-coupling of aryl neopentylglycolboronates demonstrated that they can cross-couple with aryl chlorides, bromides, and iodides with NiCl2(dppe)/dppe/K3PO4 or NaOH in dioxane at 110 °C.12a However, the same catalyst is completely inactive in the crosscoupling of aryl neopentylglycolboronates with aryl sulfonates.12a Nevertheless, preliminary results demonstrated that Ni(COD)2/PCy3/K3PO4 in THF at room temperature is an excellent catalyst for the cross-coupling of aryl mesylates and tosylates with aryl neopentylglycolboronates.3g The only other C−O-based electrophile that was cross-coupled with aryl neopentylglycolboronates is the aryl methyl ether.4 The catalyst of choice for this cross-coupling is Ni(COD)2/PCy3/CsF in toluene at 120 °C. All other C−O-based electrophiles, including esters, carbonates, carbamates, and sulfamates, were cross-coupled only with arylboronic acids by using NiCl2(PCy3)2/K3PO4 in toluene, dioxane, or xylene at temperatures ranging from 80 to 150 °C.5−8 This work was recently reviewed.2 This manuscript reports the first comparison of the efficiency of six aryl C−O-based electrophiles in cross-coupling with aryl neopentylglycolboronates. Reaction conditions specific for individual classes of C−O electrophiles namely for aryl mesylates (Ni(COD)2/PCy3/K3PO4/THF/25 °C), aryl methyl ethers (Ni(COD)2/PCy3/CsF/toluene/120 °C), aryl esters, carbonates, carbamates, and sulfamates (NiCl2(PCy3)2/K3PO4/toluene/110 °C), and a modified catalyst specific for aryl methyl ethers were investigated. Efficiency of Aryl C−O-Based Electrophiles in CrossCoupling with Aryl Neopentylglycolboronates Catalyzed by NiCl2(PCy3)2/K3PO4 in Toluene at 110 °C. The reaction conditions employed by Garg’s laboratory5a,6,8 for the cross-coupling of a diversity of aryl C−O-based electrophiles including aryl-OPiv, -OCO2NEt2, -OBoc, and -OSO2NMe2 with arylboronic acids, were used to investigate the crosscoupling of all aryl C−O electrophiles with aryl neopentylglycolboronates. These conditions employ NiCl2(PCy3)2
of aryl sulfonates and sulfamates was recently reported.16 So far, only aryl sulfonates,3g sulfamates,16 and methyl ethers4 have been shown to be active in the cross-coupling reaction with aryl neopentylglycolboronates. However, other electrophiles such as aryl pivalates, carbonates, and carbamates have not been investigated in cross-coupling with aryl neopentylglycolboronates. After a survey of the literature on C−O-based electrophiles in cross-coupling reactions, we found that the reaction conditions employed Ni(II)- or Ni(0)-based catalysts, ligands, boron sources, bases, solvents, and temperatures differed from one group of C−O electrophiles to another. The only comparative study on the efficiency of different C−O electrophiles was recently reported for cross-coupling with potassium aryl and heteroaryl trifluoroborates.3i In this paper, we report the first comparative analysis of the efficiency of six different aryl-containing C−O-based electrophiles in Ni-catalyzed Suzuki−Miyaura cross-coupling with aryl neopentylglycolboronates. These experiments were performed with four different catalytic systems that were developed specifically for the cross-coupling of aryl methyl ethers4 with aryl neopentylglycolboronates, aryl mesylates16 with aryl neopentylglycolboronates, and respectively of aryl esters, carbamates, and carbonates with arylboronic acids5a,6 and arylboroxines.5b−d
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RESULTS AND DISCUSSION Selection of the Ni-Based Catalysts. Nickel catalysis was used for the cross-coupling of aryl-containing C−O-based electrophiles with arylboronic acids, anhydrides, and boronic esters.2 Different reaction conditions and catalysts are required for the cross-coupling of aryl-containing C−O-based electrophiles with arylboronic acids, anhydrides, and boronic esters. From the entire group of C−O-based electrophiles only aryl mesylates and tosylates were investigated in cross-coupling with both boronic acids and boronic esters.2 Cross-coupling of aryl sulfonates with boronic acids proceeds in the presence of Ni(II)-based catalysts at high temperature and in the presence of Ni(0)-based catalysts at low temperature. The Ni(II)-based catalysts of choice for this reaction are NiCl2(PCy3)2/K3PO4/ 1019
dx.doi.org/10.1021/jo2022982 | J. Org. Chem. 2012, 77, 1018−1025
The Journal of Organic Chemistry
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Table 2. Cross-Coupling of 2-Naphthyl-Containing C−O Electrophiles with p-Methoxyphenyl Neopentylglycolboronate Catalyzed by Ni(COD)2/PCy3/K3PO4 in Toluene at 120 °C
a
entry
OR
time (h)
yielda (%)
1 2 3 4 5 6
OMs OSO2NMe2 OPiv OBoc OCONEt2 OMe
18 18 36 36 36 36
59 44 33 17 23 17
Isolated yield.
Table 3. Cross-Coupling of 2-Naphthyl-Containing C−O Electrophiles with p-Methyl Carboxylate Phenyl Neopentylglycolboronate Catalyzed by Ni(COD)2/PCy3/K3PO4 in Toluene at 120 °C
a
entry
OR
time (h)
yielda (%)
1 2 3 4 5 6
OMs OSO2NMe2 OPiv OBoc OCONEt2 OMe
18 18 36 36 12 36
52 49 45 24 40 22
Isolated yield.
as catalyst and flame-dried K3PO4 as base in toluene at 110 °C. The results from Table 1 showed that under these conditions aryl-OMs, -OSO2NMe2, and -OCO2NEt2 gave cross-coupling products in moderate yields (25%, 54% and 34%, Table 1, entries 1, 2, and 5). However, no reaction was observed for aryl-OBoc, and only low yields were obtained for -OPiv and -OMe (6% and 13%, respectively, Table 1, entries 3 and 6). Table 1 reports also the same cross-coupling experiments in which the K3PO4 base was replaced with CsF. With the exception of the experiments from entries 7 and 8, which showed very low efficiency, all other experiments demonstrated that CsF is not an active base for cross-couplings performed under the reaction conditions reported in Table 1. The yields of the biaryl products using aryl neopentylboronates were generally lower than those obtained for the crosscoupling of the same C−O electrophiles with arylboronic acids.6,17 It was reported that a certain amount of water facilitated the transmetalation step of the cross-coupling reaction with arylboronic acids.7a,8 However, in contrast to the reaction with arylboronic acid, no water is generated during the cross-coupling of aryl neopentylglycolboronates. Therefore, the Ni-catalysis conditions previously employed for arylboronic acids might not be the most suitable for aryl boronates. The Efficiency of Aryl C−O-Based Electrophiles in Cross-Coupling with Aryl Neopentylglycolboronates Catalyzed by Ni(COD)2/K3PO4 in Toluene at 120 °C. The catalytic system employed by the Chatani laboratory for the cross-coupling of aryl methyl ethers with aryl neopentylglycolboronates4 involving Ni(COD)2/PCy3/CsF/toluene/120 °C was modified by changing its base from CsF to
K3PO4, dried under vacuum at 40 °C overnight, while maintaining toluene as solvent at 120 °C. All aryl C−O-based electrophiles from Table 2 were cross-coupled to a certain extent under these conditions in low to moderate yield. However, the aryl -OBoc, which was previously inert under the NiCl2(PCy3)2 catalysis (Table 1, entry 4), reacted under these conditions (Table 2, entry 4). In order to explore the electronic effect of the aryl neopentylglycolboronates on the efficiency of cross-coupling, both electron-rich (Table 2) and electron-deficient (Table 3) aryl neopentylglycolboronates were studied. Under identical conditions, the electron-deficient arylboronic ester gave higher yields than the electron-rich arylboronic ester with all types of C−O-based electrophiles. Significant improvement was observed for −OCONEt2 (40%) compared to 23% in the reaction with electron-rich aryl neopentylglycolboronates. The trend of the efficiency of aryl C−O-based electrophiles was found to be: -OMs (52%) > -OSO2NMe2 (49%) > -OPiv (45%) > -OBoc (24%) > -OMe (22%) (Table 3). A similar trend was also found for Ni-catalyzed cross-coupling of C−O based electrophiles with potassium aryl and heteroaryl trifluoroborates.3i The Efficiency of Aryl C−O-Based Electrophiles in Cross-Coupling with Aryl Neopentylglycolboronates Catalyzed by Ni(COD)2/PCy3/CsF in Toluene at 120 °C. By using CsF as base at 120 °C, the C−O bond of aryl methyl ethers was successfully cross-coupled with aryl neopentylglycolboronates in moderate to good yields with Ni(COD)2/PCy3 in toluene.4 Inspired by this work, we applied these unmodified reaction conditions to all C−O-based electrophiles. The optimized conditions for the methoxy leaving group were 1020
dx.doi.org/10.1021/jo2022982 | J. Org. Chem. 2012, 77, 1018−1025
The Journal of Organic Chemistry
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The Efficiency of Aryl C−O-Based Electrophiles in Cross-Coupling with Aryl Neopentylglycolboronates Catalyzed by Ni(COD)2/PCy3/K3PO4 in THF at 25 °C. First introduced by Hu laboratory3e for cross-coupling of aryl tosylates with arylboronic acids, the catalytic system Ni(COD) 2 /PCy 3 /K 3 PO 4 in THF has been subsequently employed for the cross-coupling of aryl mesylates and sulfamates with aryl neopentylglycolboronates.3g,16 This catalytic system is very efficient for the cross-coupling of aryl sulfonates and sulfamates at room temperature regardless of the electronic properties and steric hindrances of both substrates.16 Therefore, we applied these reaction conditions to all six aryl C−O-based electrophiles. Among the C−O-based eletrophiles investigated, mesylates and sulfamates were cross-coupled with excellent yields (92−99%) (Table 5, 3a−f; OR = OMs,
found to be very specific and not applicable to other leaving groups. Aryl mesylates and sulfamates were cross-coupled with moderate yields (36% to 61%) (Table 4, 3b and 3d; OR = Table 4. Cross-Coupling of Aryl Containing C−O Electrophiles with Para-Substituted Aryl Neopentylglycolboronates Catalyzed by Ni(COD)2/PCy3/ CsF in Toluene at 120 °C
Table 5. Cross-Coupling of Aryl-Containing C−O Electrophiles with Para-Substituted Aryl Neopentylglycolboronates Catalyzed by Ni(COD)2/PCy3/ K3PO4 in THF at 25 °C
OMs, OSO2NMe2). These results are in agreement with the results obtained when NiCl2(PCy3)2/K3PO4 was used in toluene at 110 °C (Table 1, entries 1 and 2) and also Ni(COD)2/PCy3/K3PO4 in toluene at 120 °C (Table 2, entries 1 and 2). In addition, cross-coupling of aryl carbamates gave poor yields (13−14%, Table 4, 3b and 3d; OR = OCONEt2) while -OPiv was almost inert (
